Semiconductor light emitting device and method for manufacturing the same

ABSTRACT

A first GaN layer ( 2 ) is formed on a substrate ( 1 ), mask layer ( 3 ) having opening parts ( 3   a ) are formed thereon, a second GaN layer ( 4 ) is selectively grown in the lateral direction from the opening parts on the mask layer, and further a nitride type compound semiconductor layered part ( 15 ) is so laminated as to form a light emitting layer. Recessed parts ( 3   b ) are formed in the upper face side of the mask layer. In other words, owing to the recessed parts in the upper face side of the mask layer, the second GaN type compound semiconductor layer ( 4 ) is grown as to form approximately parallel gap ( 3   c ) between the bottom face of the second GaN type compound semiconductor layer and the mask layer. Further, it is preferable for the mask to be formed in a manner that the opening parts for exposing the seeds are not arranged only continuous in one single direction in the entire surface of the wafer type substrate. Consequently, a nitride type compound semiconductor light emitting device can be obtained while being provided with a low dislocation density and excellent light emitting efficiency and especially a semiconductor laser with a lowered threshold current value can be obtained.

This application is a division of application Ser. No. 09/864,275, filedMay 25, 2001, now U.S. Pat. No. 6,469,320.

FIELD OF THE INVENTION

The present invention relates to a semiconductor light emitting devicesuch as a semiconductor laser, a light emitting diode or the like and afabrication method thereof, which uses a nitride-based compoundsemiconductor (a compound semiconductor of group III element(s) andnitrogen and the like), and is capable of emitting light in the bluecolor region required for an optical disk memory having a high memorydensity or improving delicacy of a laser beam printer. Moreparticularly, the present invention relates to a semiconductor lightemitting device such as a semiconductor laser and a fabrication methodthereof capable of preventing warp in a wafer while suppressing thethreading dislocation (defect) density of a nitride-based compoundsemiconductor layer as much as possible by employing epitaxial lateralovergrowth and of improving the electroluminescent properties.

BACKGROUND OF THE INVENTION

A conventional light emitting diode (LED) or laser diode (LD) emittinglight in a blue-emitting region has been fabricated by successivelyforming compound semiconductor of group III element nitrides on asapphire substrate by Metal Organic Chemical Vapour Deposition(hereinafter referred to as MOCVD).

For example, a semiconductor laser capable of carrying out CWoscillation in a blue-emitting region is fabricated as shown in FIG. 10by successively forming layers of group III element nitride typecompound semiconductor on a sapphire substrate 21 by the MOCVD method; aGaN buffer layer 22, a contact layer 23 of an n-type GaN, an n-type cladlayer 24 of Al_(0.12)Ga^(0.88)N, an n-type light guide layer 25 of GaN,an active layer 26 of an InGaN based (type) compound semiconductor withmultiple quantum well structure, a p-type light guide layer 27 of ap-type GaN, a p-type clad layer 28 of a p-type Al_(0.12)Ga_(0.88)N, anda p-type contact layer 29 of a p-type GaN; etching some of the layeredsemiconductor layers as shown in FIG. 10 by, for example, dry etching toexpose the n-type contact layer 23, and forming an n-side electrode 31thereon and a p-side electrode 30 on the foregoing p-type contact layer29, respectively. The portion of the p-side electrode 30 along thestripes is utilized as the light emitting part.

However, the sapphire substrate on which the nitride based compoundlayers are grown has considerably different lattice constant and thermalexpansion coefficient from those of the nitride type compoundsemiconductor layers and the density of the threading 25 dislocation(TD) of the nitride based compound semiconductor layers grown thereon isas high as about 1×10⁸cm⁻² to 1×10¹⁰cm⁻² and the dislocation density issignificantly high as compared with that, 1×10²cm⁻², of compoundsemiconductor layers of the red-emitting type grown on GaAs substrate.In case of LEDs (light emitting diode), even if there occurs dislocationdensity about that level, a compound semiconductor is practicallyapplicable, however in case of semiconductor lasers, if the dislocationdensity is especially high, the threshold current is increased, so thatit is desired to lower the dislocation density at highest to about1×10⁷cm⁻² or lower in order to obtain a low threshold value and a longlife. However, other than sapphire, any alternative substrate suitablefor industrial use has not been found.

On the other hand, as a technique to lower the TD, crystal growth usingELO (epitaxial lateral overgrowth) has drawn attention as crystal growthmethods, as disclosed in for example, “Thick GaN Epitaxial Growth withLow Dislocation Density” by Akira Usui et al. (Jpn. J. Apply. Phys.,vol. 36, 1997, pp. 899-902) and “ELO growth of GaN by hydride VPE andMOVPE” by Sasaoka et al. (Jpn. J. Crystal Growth, vol. 25 no. 8, 1998,pp. 99-105).

These methods are methods including, for example, steps of putting aSiO₂ mask 43 having opening parts 44 on a first GaN layer 42 on asapphire substrate 41 and growing a second GaN layer 45 on the SiO₂ mask43 by selectively growing the layer in the lateral direction using thesemiconductor layer exposed to the opening parts 44 as a seed as itssectional explanatory view is partially shown in FIG. 11 and prevent TDbased on that a nitride type compound is easier to be grown in thelateral direction than in the vertical direction. The literature citedin the former journal discloses that a mirror face GaN layer free fromcracks and having the dislocation density of lower than 6×10⁷cm⁻² can begrown on a sapphire wafer with the diameter of 2 inch by the forgoingmethod.

However, in case of ELO growth, as shown in FIG. 11, although the secondGaN layer 45 is so grown successively in the lateral direction fromopening parts 44 in both sides formed at constant intervals in the masklayer 43 on the first GaN layer 42 as to meet in the center part of themask layer 43, the second GaN layer 45 growing on the mask 43 tends tobe lifted out of the mask 43 as it goes to the center part side and isgrown while the crystallographic axis being curved to result in that thesecond GaN layer 45 cannot have a flat bottom face and surface.Therefore, as shown in FIG. 11, a void 46 is formed owing to the join ofthe second GaN layer 45 in the center part side of the mask while thesecond GaN layer 45 being lifted out and it is undesirable to fabricatea device using the resultant wafer. Such tendency further becomesignificant if the mask width M becomes wide.

In order to avoid deterioration of the flatness, for example, as it canbe understood from the case of the method cited in the literature of theabove mentioned former journal where the mask width M of the SiO₂ maskis 1 to 4 μm and the cycle (M+W) is about 7 μm, the mask width M isrequired to be narrow since the void 46 is easily formed if the maskwidth M is 3 μm or wider. Moreover, as the width M becomes wider, theheight and the size of the void become high and large andconsequently,the flatness of the surface is deteriorated to result ininferiority of device properties. Further, even if the ultimateconditions in which no void 46 is formed and the flatness is barelykept, the dislocation density is increased in the join part in thecenter. Furthermore, the second GaN layer 45 growing in the openingparts 44 also has threading dislocation continuous as it is from thefirst GaN layer 42 in the vertical direction to become a region with ahigh dislocation density. Therefore, the continuous parts with a smalldislocation density are only obtained in a half of the mask width andmore precisely in portions excluding both end parts of the half andwithin about 1 μm by width.

Nevertheless, in case of using the obtained wafer for a stripe typesemiconductor laser, the light emitting part of the laser is only someregion which is stripe-like and therefore it is supposed to be possiblefor the semiconductor laser to suppress the increase of thresholdcurrent value attributed to the crystal defects and the deterioration ofelectric operating properties of LD by lowering the dislocation densityof the corresponding part of the semiconductor layer. In this case, asshown in FIG. 9, although it is effective to reciprocally form theopening parts 44 and mask layer 43 linearly in one direction, thedislocation density becomes high as described above in the both endparts and the center parts in the mask width and it is thereforepreferable to use the portion excluding both end parts in a half of themask width for a light emitting region with a stripe width of LD. Hence,supposing the stripe width of the LD to be 4 to 5 μm, the mask width Mis required to be at least 10 to 15 μm and in this case, the GaN to begrown on the mask is required to be grown thicker than the mask width,that is 15 to 20 μm.

As described before, in order to fabricate a semiconductor laser devicewith few crystal defects even only in the light emitting part of thedevice, it is required to form opening parts in the mask along thestripe direction in which the light emitting part of the device isformed and as shown in FIG. 9, the opening parts are formed in the maskonly in one direction. On the other hand, there occurs no problem incase of a thin layered structure comprising a mask with a narrow widthand the GaN layer with about 5 μm thickness grown on the mask, howeverif the mask width is 10 μm or wider and the thickness of a semiconductorlayer to be layered thereon is 15 μm or thicker, the warp of a waferbecomes significant attributed to the difference of the thermalexpansion coefficient between the semiconductor layer and the substrate.In the case where the wafer is warped, uniform treatment in the wafercannot be carried out during the wafer process, resulting in problemsthat the stripe width cannot be even, cracking easily takes place at thetime of wrapping the wafer, and that properties are easy to be variedowing to the effect of the stress on the device by the warp.

On the other hand, if the thickness of the substrate is made as thick asabout 700 μm from the conventional thickness, about 330 μm, suchproblems in the wafer process are eliminated, however there occursanother problem that the final polishing of the substrate before thesubstrate is made a chip requires a burdensome work and also if thesubstrate is polished the foregoing warping problem takes place.Further, Japanese Unexained Patent Publication No. Hei 11-186178 gazettediscloses a method to prevent the wafer from warping owing to thedifference of the thermal expansion coefficient from that of thesubstrate by employing the foregoing ELO growth and forming the grownregions like islands without carrying out epitaxial growth of asemiconductor layer on the entire surface of the substrate (no growthtakes place by forming a large mask in a non-growth region). However, bythis method, at least a half of the wafer is the non-growth region asexamples show to leave a problem that the wafer is utilized considerablyin vain.

SUMMARY OF THE INVENTION

The present invention has been developed in the above describedsituation and a first object of the present invention is to provide anitride type compound semiconductor light emitting device with a highlight emitting efficiency by selectively forming a nitride type compoundsemiconductor layer with flatness in a wide range on a mask of SiO₂ orthe like while suppressing the dislocation density.

Another object of the present invention is to provide a semiconductorlaser capable of providing high outputs by suppressing the dislocationdensity of the active layer in at least stripe-like light emittingregion and lowering the threshold current value in the case where thelight emitting region can be restricted within the stripe-like part justas the case of a semiconductor laser.

Further another object of the present invention is to provide asemiconductor laser with a structure possible to prevent warp of a waferwhile suppressing the dislocation density of the active layer in suchstripe-like light emitting region.

Further another object of the present invention is to provide a methodfor fabricating a semiconductor light emitting device by which the warpof a wafer is suppressed to the extent that there occurs no problem inthe device properties and the handling of the wafer while the threadingdislocation density of the entire wafer being suppressed by widening themask width in the case where a semiconductor layer is selectively grownon the mask by ELO growth.

Inventors of the present invention have enthusiastically investigated inorder to solve a problem that a growing nitride type compoundsemiconductor layer is grown while the crystallographic axis beingwarped more upward as it goes closer to the center part of a mask toleave a void in the periphery of the center part in case of selectivegrowth of the semiconductor layer on the mask in the lateral directionand that the void becomes bigger as the mask width is wider to inhibitgrowth of the flat semiconductor layer, and inventors have found thereason why the upward warping of the crystallographic axis of thegrowing semiconductor layer is getting more significant as thesemiconductor layer is grown closer to the center part side of the maskis attributed to the contact stress affecting the contact parts of thesemiconductor layer and the mask layer. Inventors have also found that aflat semiconductor layer with a small dislocation density can be grownwithout being accompanied with the warp of the crystallographic axis byparting the contact parts and contact stress is prevented fromaffecting.

A semiconductor light emitting device according to the present inventioncomprises a substrate, a mask layer having opening parts and formeddirectly on the substrate or on a layer formed on the substrate, anitride type compound semiconductor layer selectively grown in thelateral direction on the mask from the opening parts, and asemiconductor layered part comprising nitride type compoundsemiconductor layers so formed on the nitride type compoundsemiconductor layer as to form light emitting layer, wherein the masklayer is provided with at least one recessed part on the upper faceside, or the foregoing second nitride type compound semiconductor layerhas a flat face in the bottom face side and is so grown as form anapproximately parallel gap between to the bottom face of the secondnitride type compound semiconductor layer and the mask layer.

In this case, the nitride type compound semiconductor denotes asemiconductor of a compound of group III elements such as Ga, Al, In andthe like, and N and other elements of group V elements. Consequently,the compound semiconductor means N-containing compound semiconductorwith properly changed mixed crystal ratio of the group III elements andof the group V elements, such as AlGaN type compound in which thecomposition ratios of Al and Ga can be changed, InGaN type compound inwhich the composition ratio of In and Ga can be changed, other than GaN.Further, the mask layer means a layer made of a material, for example,SiO₂, on whose surface an epitaxial growth of a nitride type compoundsemiconductor layer cannot directly be carried out in case of trying theepitaxial growth.

According to such a structure, since recessed parts are formed in themask layer to be an underlayer of the second nitride type compoundsemiconductor layer selectively grown in the lateral direction, or thesecond nitride type compound semiconductor layer is formed as to keep agap to the mask layer, a second gallium nitride type compoundsemiconductor layer to be grown is not affected with the stress from themask layer. As a result, the crystallographic axis of the second galliumnitride type compound semiconductor layer is prevented from upwardwarping as the compound semiconductor layer is growing in the lateraldirection and the semiconductor layer is straightly grown in the lateraldirection in a wide width to obtain the second nitride type compoundsemiconductor layer excellent in flatness and having an extremely lowdislocation density. Further, since the semiconductor layered part ofthe nitride type compound layered thereon is grown on the semiconductorlayer with a low dislocation density, it is made possible to form thesemiconductor layered part with excellent flatness and a low dislocationdensity.

The layer to be formed on the foregoing substrate may be a first nitridetype compound semiconductor layer and the foregoing nitride typecompound semiconductor layer to be formed on the foregoing mask layermay be a second nitride type compound semiconductor layer.

A semiconductor laser of the present invention is the foregoing nitridetype compound semiconductor light emitting device in the above describedin claim 1 or 2, wherein the semiconductor layered part is laminated soas to constitute a semiconductor laser structure, the mask layer has apart extended like stripe by being sandwiched between neighboringopening parts, the recessed part or the gap is formed in a prescribedwidth along the stripe in the part extended like stripe, and thesemiconductor layered part is so formed that a current injection regionin stripe is formed in a constant width within a half of the prescribedwidth. By constituting such a structure, without requiring semiconductorlayers with a low dislocation density to be formed in a wide range, thesemiconductor layered part necessary in light emitting region in stripecomprises only layers with a low dislocation density and can be formedas to have excellent flatness and it is thus made possible to obtain asemiconductor laser with a small threshold current value, a high outputand excellent reliability.

Further, inventors of the present invention have made investigation inorder to eliminate the effect of the wafer' warp attributed to the widewidth of the mask and the thick thickness of the nitride type compoundsemiconductor layer grown thereon in case of selective growth (ELOgrowth) of the nitride type compound semiconductor layer on the mask inthe lateral direction and consequently found that if opening parts ofthe mask layer are formed continuously only in one direction, the warptakes place in a constant direction in relation to the opening parts andthat, on the contrary, if most parts of the opening parts are dispersedor the opening parts are not formed to be continuous only in onedirection, the warp can significantly be suppressed even if thethickness of the growing layer is made thick by widening the mask widthor even if long and straightly linear opening parts are formed.

A method for manufacturing a semiconductor light emitting deviceaccording to the present invention comprises the steps of; forming amask layer, on which a nitride type compound semiconductor layer cannotbe formed directly, either directly on the surface of a wafer-typesubstrate or on a layer formed on the substrate; forming opening partsfor exposing seeds to grow nitride type compound semiconductor layerlayers in the mask in a manner that the opening parts are not arrangedonly continuous in a single direction in the entire surface of theforegoing wafer-type substrate; forming a nitride type compoundsemiconductor layer on the entire surface of the foregoing wafer-typesubstrate by selectively growing the layer on the foregoing mask layerfrom the opening parts in the lateral direction; forming a semiconductorlayered part comprising nitride type compound semiconductor layers as toform light emitting layer on the foregoing nitride type compoundsemiconductor layer; and producing chips from the resultant wafer typesubstrate.

According to the present method, since the opening parts of the masklayer are not formed continuously only in one direction, the stressowing to the difference of the thermal expansion coefficient between thesubstrate and the semiconductor layers grown thereon does not affectonly in one direction but evenly applied in every direction to result inprevention of significant warp in only one direction. Consequently,treatment unevenness in wafer processing process and cracking of thewafer can be avoided to considerably improve the quality of theresultant product.

Most of the foregoing opening parts are formed to be a rectangular orhexagonal shape in a plan view, so that the length in the growthdirection can be easy to be adjusted even if the speed of the crystalgrowth in the lateral direction is different between the directionperpendicular to the A face and the direction perpendicular to the Mface in case of the rectangular shape and the growth speeds in therespective directions can be made equal one another due to that a GaNtype compound is of a hexagonal system in case of the hexagonal shape,so that the growth in the lateral direction can be made even. That mostof the opening parts are formed to be a rectangular or hexagonal shapein this case includes that straightly linear opening parts formedpartially, for example, in the peripheries of the stripe-like lightemitting parts of an LD.

Another embodiment of the semiconductor laser according to the presentinvention is a semiconductor laser comprising; a substrate, a mask layerhaving opening parts and formed directly on the substrate or on a layerformed on the substrate, a nitride type compound semiconductor layerselectively grown in the lateral direction on the mask layer from theopening parts, and a semiconductor layered part comprising nitride typecompound semiconductor layers so formed on the nitride type compoundsemiconductor layer as to form a light emitting layer having a stripetype light emitting part, wherein the mask layer has a part extended inthe entire chip length with no opening part transversely crossing thelower side of the stripe type light emitting part and has the openingparts dispersedly in the parts other than the lower side of the stripetype light emitting part.

Owing to such a structure, the stripe type light emitting part of thesemiconductor laser are composed of only parts with extremely fewcrystal defects and since the opening parts of the mask layer are notformed only in one direction, the warp of the wafer can be suppressed toa significantly low level and the production yield is improved and atthe same time the obtained semiconductor laser is provided withexcellent properties and a low threshold current.

More practically, the opening parts of the mask layer are formedlinearly along the stripe type light emitting part in the portionsadjacent to the stripe type light emitting part and formed in amatrix-like form or randomly in the portions other than the portionsadjacent to the stripe type light emitting part or the mask layer isformed into a shape symmetric three times by layering a first patternhaving linear opening parts, a second pattern obtained by rotating thefirst pattern at 60°, and a third pattern obtained by rotating the firstpattern at 120°, so that the opening parts of the mask layer are notrestricted only to those continuous only in one direction and thesemiconductor laser having the stripe type light emitting part can beformed by semiconductor layers with a low dislocation density entirelyin the stripe type light emitting part.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a explanatory sectional view of one embodiment of asemiconductor laser according to the present invention.

FIG. 2 is a explanatory sectional view of mask layer part of FIG. 1 inthe wafer state.

FIG. 3 is a explanatory view at the time of forming a recessed part inthe mask layer of FIG. 1.

FIGS. 4(a) and 4(b) are explanatory views of pattern examples formed byetching the semiconductor layered part after laminating thesemiconductor layers.

FIG. 5 is a graph showing the alteration of the threshold current of thesemiconductor laser of the present invention in relation to thedislocation density.

FIGS. 6(a) and 6(b) are examples of mask pattern to be employed for amethod of fabricating another embodiment of a semiconductor lightemitting device according to the present invention.

FIG. 7 is another example of a mask pattern suitable for a semiconductorlaser according to the present invention.

FIG. 8 is further another example of a mask pattern suitable for asemiconductor laser according to the present invention.

FIG. 9 is an example of a pattern preferable for growing a semiconductorlayer with a small dislocation density in the entire body of a stripetype light emitting part of an LD and a view illustrating the warpingstate in this case.

FIG. 10 is a explanatory sectional view showing one example of aconventional blue-emitting type semiconductor laser.

FIG. 11 is a explanatory view showing the correlation between mask layerand opening parts in case of selective growth in lateral direction usinga conventional mask layer.

DETAILED DESCRIPTION

Next, a semiconductor light emitting device of the present inventionwill be described with the reference to FIGS. As shown in FIG. 1 as aexplanatory view of a semiconductor laser, which is one embodiment, thesemiconductor light emitting device according to the present inventioncomprises a first nitride type compound semiconductor layer 2 on asubstrate 1, mask layer 3 having opening parts 3 a formed thereon, asecond nitride type compound semiconductor layer 4 formed on the masklayer 3 by being selectively grown in the lateral direction from theopening parts 3 a, and a semiconductor layered part 15 composed oflayered nitride type compound semiconductor layers as to form lightemitting layer. Recessed parts 3 b are formed on the upper face side ofthe mask layer 3. In another structure, owing to the recessed parts 3 bon the upper face side of the mask layer 3, the second nitride typecompound semiconductor layer 4 is so formed to be a flat face in thebottom face side and the second nitride type compound semiconductorlayer 4 may be grown as to form a gap 3 c approximately parallel betweenthe bottom face of the second nitride type compound semiconductor layer4 and the mask layer 3.

As the substrate 1, for example, a sapphire (Al₂O₃ single crystal)substrate capable of standing a high temperature can be employed,however not only sapphire but also other semiconductor substrates ofsuch as Si, Ge and the like can be employed. In any case of using anyone of the materials, the lattice constant is not equal to that of GaN,so that lattice conformation cannot be achieved. However, selectivegrowth in the lateral direction through the mask layer makes it possibleto grow a semiconductor layer with a small dislocation density on themask layer.

The first nitride type compound semiconductor layer 2 is formed, forexample, by a common epitaxial growth of a non-doped GaN in about 4 μmby a MOCVD method and is to be employed as a seed at the time ofselective growth of the second nitride type compound semiconductor layer4, which will be described later. Nevertheless, if it is possible tocarry out nitride type compound semiconductor layer growth using asemiconductor substrate of such as Si or a sapphire substrate as a seedeven in the case where the first nitride type compound semiconductorlayer is not formed, the first semiconductor layer can be omitted.

The mask layer 3 is formed in about 200 μm thickness by sputtering amaterial, for example, SiO₂, Si₃N₄, W or the like, on which asemiconductor layer cannot directly epitaxially be grown. The mask layer3 is for avoiding the second semiconductor layer from directly beingformed on the surface of the substrate 1 or the first GaN layer 2 and aslong as it is formed as to satisfactorily perform the function as amask, the mask layer is more preferable to be thinner since steps arehardly formed if it is thin. As shown in FIG. 2 showing a partialexplanatory sectional view of wafer state, the mask layer 3 is providedwith the opening parts 3 a by patterning after it is formed on theentire surface of the first nitride type compound semiconductor layer 2in the wafer state and further with recessed parts 3 b formed along theopening parts 3 a in the surface side of the remaining mask layer 3.

In the case of fabricating a semiconductor laser shown in FIG. 1, thewidth W of the opening parts 3 a is about 10 to 20 μm and the width M ofthe mask layer 3 is about 20 μm. According to the present invention,even if the mask width, about 20 μm, is widened, the secondsemiconductor layer 4 with a flat surface can be formed. In FIG. 1,since the stripe parts and the mask layer 3 are shown being magnified,only a part of mask layer 3 is shown, however actually a large number ofparts of mask layer 3 are formed in one chip in reciprocal foregoing Mand W intervals.

As shown in FIG. 3 showing the explanatory sectional view of a recessedpart at the time of formation thereof, the recessed parts 3 b formed onthe surface of the mask layer 3 are formed to have the depth d about ahalf thickness t of the mask layer 3, in other words about 100 nm depthd, by forming a resist film 18 in the entire surface on the mask layer 3after the opening parts 3 a are formed, forming opening parts 18 a withthe width N of about 16 μm in the resist film 18 by patterning, and thencarrying out etching with an aqueous HF type solution. Consequently,while leaving the portions about 2 μm width P from both end parts of themask layer 3, the recessed part 3 b are formed on the surface and in theinner side.

The width P left in both ends are adjusted to be about 2 μm inconsideration of the precision of the mask alignment and it is left inorder to prevent growth continuously to the insides of the recessedparts 3 b from the opening parts 3 a and therefore, it is sufficient tokeep the growth starting position in the lateral direction at a higherposition than most parts of the surface of the mask layer 3 and to keepa gap between most parts of the surface and the second semiconductorlayer 4 to be grown in the lateral direction. For that, other than therecessed parts 3 b formation, any structure, e.g. projections formed inthe opening parts 3 a side, can be applicable as long as the structureis suitable to keep a gap approximately parallel to the secondsemiconductor layer 4. Also, the depth of the recessed parts 3 b (theheight in case of forming projections in both end parts) is sufficientif it is enough to form steps to prevent contact stress from affectingbetween the mask layer and the second semiconductor layer 4 to be grownin the lateral direction. Due to that, the recessed parts 3 b may beformed in the depth to the extent that the second semiconductor layer 4is allowed to be slightly formed in the bottom face side and formed onthe almost surface of the mask layer 3 in the ultimately allowablepositioning relation without leaving gaps, however taking the dispersionin fabrication conditions, the depth is preferable to be about 100 nm asdescribed before.

The second nitride type compound semiconductor layer 4 is, for example,an about 20 μm-thick non-doped GaN layer. The semiconductor layer 4 isstarted growing while using the first GaN layer 2 exposed to the openingparts 3 a of the mask layer 3 as a seed and selectively grown in thelateral direction when it reaches the surface of the mask layer 3. Thatis, since GaN layer is grown faster with more excellent crystallinity inthe lateral direction than in the vertical direction, although therecessed parts 3 b are formed in the mask layer 3, the layer is scarcelygrown in the lower side but grown in the lateral direction while keepinggaps 3 c to the mask layer 3 and also grown slightly in the upper sideand finally joined to itself around the center parts of the mask layer 3while being grown in the lateral direction in both ends of each openingpart. Then, after the surface of the mask layer 3 are completely buried,the second GaN layer(a semiconductor layer) 4 is grown upward tocompletely cover the mask layer 3. The resultant second GaN layer 4 hasexcellent crystallinity in the portions other than the portions in bothend parts of the mask layer 3 (the portions contacting the opening parts3 a) and the joining parts in the center parts and the dislocationdensity is lowered by one figure.

As shown in FIG. 1, the semiconductor layered part 15 on the second GaNlayer 4 is so formed to be a semiconductor layered part as to constitutea common semiconductor laser structure. That is, the layered part 15 isformed by successively laminating an about 0.5 μm thick n-type contactlayer 5 of n-type GaN doped with, for example, Si of about 5×10¹⁸cm⁻³,an about 0.4 μm thick n-type clad layer 6 of n-type Al_(0.08)Ga_(0.92)Ndoped with, for example, Si of about 5×10⁸cm⁻³, an about 0.2 μm thickn-type first guide layer 7 of n-type GaN doped with, for example, Si ofabout 1×10¹⁸cm⁻³, an about 50 nm thick n-type second guide layer 8 ofn-type In_(0.01)Ga_(0.99)N doped with, for example, Si, an about 50nm-thick active layer 9 having a multiple quantum well (MQW) structurecomprising 5 well layers of about 5 nm-thick In_(01.)Ga_(0.9)Nsuccessively and reciprocally stacked on about 5 nm-thick barrier layersof In_(0.02)Ga_(0.98)N, an about 20 nm thick p-type cap layer 10 ofAl_(0.2)Ga_(0.8)N doped with, for example, Mg, an about 0.1 μm thickp-type guide layer 11 of GaN, doped with, for example, Mg of about1×10¹⁸cm⁻³, an about 0.4 μm thick p-type clad layer 12 ofAl_(0.08)Ga_(0.92)N doped with, for example, Mg of 2×10¹⁷cm⁻³, and anabout 0.1 μm thick p-type contact layer 13 of GaN doped with, forexample, Mg of about 3×10¹⁸cm⁻³.

The structure of the semiconductor layered part 15 and the material foreach layer are not at all restricted to those exemplified above and theactive layer 9 may not be a quantum well structure but be a bulkstructure and the active layer 9 of a material determined by the desiredwavelength of emitted light is so composed as to be sandwiched betweenclad layers 6, 12 made of materials having band gaps wider than that ofthe active layer 9. In case of composing a semiconductor laser as shownin FIG. 1, the active layer 9 is made of a material having a refractiveindex higher than those of the materials for the clad layers 6, 12.Owing to that, light can be closed in the active layer 9 and if theactive layer 9 is thin and cannot sufficiently close the light, as shownin FIG. 1, light guide layers 7, 8, 11 having refractive indexes betweenthat of the active layer 9 and those of the clad layers 6, 12 areformed. However, if the active layer 9 is capable of sufficiently closelight, it is no need to form the light guide layers 7, 8, 11.

The p-type contact layer 13, the uppermost layer of the semiconductorlayered part 15, is subjected to mesa-etching, and a part of thesemiconductor layered part 15 is etched to expose the n-type contactlayer 5, and a SiO₂ film is formed on the entire surface to form theprotective film 14. After that, the p-side electrode 16 of Ni—Au isformed on the mesa part of the p-type contact layer 13 through thecontact hole of the protective film 14 and the n-side electrode 17 ofTi—Al is formed as being electrically connected with the n-type contactlayer. Finally, the resultant layered structure is cleaved as to haveresonance length of 500 μm (the length in the perpendicular direction tothe sectional plane of FIG. 1) and compose the laser (LD) chip shown inFIG. 1.

With this layered structure, the mesa-type parts in stripes of thep-type contact layer 13 become current injection regions (in the casewhere the p-side electrode is formed in stripes, stripe type currentinjection region is formed even if the contact layer 13 is not formed tobe mesa type) and the mask layer 3 and the p-side electrode 16 areformed while being conformed to each other as to position a half or lessof the stripe type recessed part 3 b formed on the mask layer 3.

According to the present invention, in the case where the nitride typecompound semiconductor layer is grown by selective growth in the lateraldirection on the mask layer, since the recessed parts are formed in thesurface of the mask layer, even if the semiconductor layer is grown onthe mask layer by selective growth of the semiconductor layer, the layergrowth is promoted in the lateral direction while leaving gap to themask layer and therefore, no contact stress affect between the masklayer and the semiconductor layer during the growth. Consequently, thecrystallographic axis of the grown semiconductor layer is prevented frombeing curved by the stress and a flat semiconductor layer is grown in along width. (Even if a gap is not formed, formation of the recessed partprevents the contact stress from affecting between the selectively grownsemiconductor layer and the mask layer.) Further, since the growth is inthe lateral direction, the dislocation density is as low as 5×10⁶cm⁻²,which is low by one order as compared with that in a conventional one toform a semiconductor layer with excellent crystallinity and flatness ina wide range.

As shown in the example of FIG. 1, the recessed part to be formed in themask layer is formed in stripe and the semiconductor layered part isformed on them within a half width of the recessed part as to formcurrent injection region, so that light emission is enabled to takeplace only in the portions of the semiconductor layered part withsignificantly excellent crystallinity and flatness. So, even ifsemiconductor layered part with excellent crystallinity and flatnesscannot be formed in the entire surface of a wide range, a semiconductorlaser with a low threshold current value and a high oscillation outputcan be obtained. That is, as the correlation of the dislocation densityand the threshold current value is shown in the graph of FIG. 5 of thesemiconductor laser with the structure of FIG. 1, the dislocationdensity is lowered to 5×10⁶cm⁻² from 2×10⁸cm⁻² and the threshold currentvalue is also lowered to 5 kA/cm² from 10 kA/cm² according to thepresent invention.

In other words, even in the selective growth in the lateral directionusing a mask, the first semiconductor layer to be a seed has inferiorcrystallinity and a high dislocation density in the opening parts of themask, so that the semiconductor layer to be grown thereon also has ahigh dislocation density and inferior crystallinity. Further, if thewidth of mask layer is wide, it is harder to keep the flatness as itgoes closer to the center part of the mask layer and the crystallinityof the semiconductor layer in the joining portion grown from both sidesof the opening parts is lowered to make it difficult to obtain thesemiconductor layer with excellent crystallinity and flatness in theentire face of a wide surface area. However, by adopting the foregoingconstitution, the stripe-like resonator portion, which is the lightemitting portion of the semiconductor laser, can be grown on thesemiconductor layer with excellent crystallinity and flatness to make itpossible to obtain the semiconductor laser comprising the semiconductorlayered part with excellent crystallinity grown in the resonator portionand provided with a low threshold current value.

Next, the method for fabricating (manufacturing) such a semiconductorlaser will be described. Using an epitaxial growth apparatus by, forexample, MOCVD method, a substrate is thermally cleaned in H₂ gasatmosphere at 1100° C. substrate temperature. After that,triethylgallium (TEG) as a Ga raw material gas and ammonia (NH₃) as a Nraw material gas are introduced to grow a non-doped first GaN layer 2 inabout 4 μm thickness. Then, the resultant substrate is taken out thegrowth apparatus and using, for example, a sputtering apparatus, a SiO₂film in about 200 nm thickness is formed thereon. After that, a resistlayer is formed on the SiO₂ film, patterned, and the SiO₂ film is etchedusing an aqueous HF solution to form stripe type opening parts and toform stripe type mask layer 3. Moreover, as shown in FIG. 3, a resistfilm 18 is formed on the entire surface and patterned to form opening inthe portion where the recessed parts 3 b is to be formed. Then, etchingwith an aqueous HF solution is again carried out to form stripe typerecessed part 3 b (in the perpendicular direction to the sectional planeof FIG. 3) as shown in FIG. 3.

After that, the resultant substrate is again put in a growth apparatussuch as a MOCVD apparatus and necessary gases such as trimethylaluminum(TMA) and trimethylindium (TMIn) as raw material gas of Al and In,respectively, as well as the foregoing gases, SiH₄ as the n-type dopantor cyclopentadienyl magnesium (Cp₂Mg) or dimethyl zinc (DMZn) as p-typedopant are introduced together with carrier gas to grow the second GaNlayer 4 and respective semiconductor layers of the semiconductor layeredpart 15 in respectively above described thickness. In this case, thefirst n-type guide layer 7 and those before the layer 7 are grown at thesubstrate temperature of 1050° C. and the second n-type guide layer 8and the active layer 9 are grown at the substrate temperature of 770° C.and the respective layers after these are grown again at the substratetemperature of 1050° C.

On completion of the growth of the respective semiconductor layers, thesubstrate is taken out of the growth apparatus and a resist mask is puton the surface, a part of the semiconductor layered part 15 is etched bya Reactive Ion Beam Etching (RIBE) apparatus in 200 μm width in thecycle of 400 μm as shown in FIG. 4(a) to expose some of n-type contactlayer 5. Further, the resist mask is removed and then another resistmask is put again, mesa etching is carried out by the same apparatus asto leave the p-type contact layer 13 in about 4 μm width as shown inFIG. 4(b). After that, using a film growth apparatus such as a plasmaCVD, a protective film 14 of such as SiO₂ is formed in about 200 nmthickness on the entire surface and the parts where electrodes are to beformed are etched by a HF-type etchant to form contact holes.

After that, as the p-side electrode 16, a Ni film of 100 nm thicknessand a Au film of 200 nm thickness are formed respectively by a vacuumevaporation apparatus and further as the n-side electrode 17, a Ti filmof 100 nm thickness and an Al film of 200 nm are formed to obtain theelectrodes 16, 17 and the rear side of the resultant substrate 1 ispolished to be as thin as about 60 μm and cleave as to keep theresonance length of about 500 μm to complete a LD chip.

Although the foregoing exemplified one is a semiconductor laser with thestripe structure in which the p-type contact layer 13 is simply mesatype stripes, it may be sufficient to form only the p-side electrode instripe without etching the contact layer or to form mesa type structurealmost near to the active layer or to form a proton implanted typestructure by implanting protons. Further, the semiconductor laser may beformed in an index guiding structure in which the current blocking layeris buried. Further, although the foregoing example is an example of thesemiconductor laser, even in case of a light emitting diode (LED), asemiconductor layer with excellent crystallinity can be formed in a widerange according to the present invention and even if there are someportions where the dislocation density is high, the ratio of theportions in the entire light emitting part is low, so that the lightemitting efficiency is improved.

According to the present invention, a semiconductor layer with a lowdislocation density can be formed in a wide range and in the case, asemiconductor laser is formed by forming the opening parts in a mask asdescribed above, there is apt to occur a problem that a wafer is easy tobe warped. The following is the description of a method for fabricatinga semiconductor light emitting device and a semiconductor laser whilesolving the problem.

A semiconductor light emitting device and its fabrication method ofanother embodiment of the present invention is characterized in that, inthe case where the semiconductor layer is deposited by selectivelygrowing a nitride type compound semiconductor on a wafer type substrateby the ELO growth and composing light emitting layer part thereon, themask pattern for selective growth in the lateral direction is so formedas to have most of the opening parts symmetric n (the referencecharacter n denotes an integer of 2 or higher) times (that means thestructure becomes symmetric n times in rotation at 180°) as the exampleof the mask pattern of one embodiment shown in FIG. 6(a), but not formedas to have opening parts exposing the seed to be only a patterncontinuous in one single direction in the entire surface of the wafertype substrate.

Using such a mask, a nitride type compound semiconductor layer is formedon the entire surface of the wafer type substrate by carrying outselective growth in the lateral direction from the opening parts on themask layer and nitride type compound semiconductor layers are stacked asto form light emitting layer and form semiconductor layered part andthen the resultant substrate is cut and broken into chips to obtain asemiconductor light emitting device.

The example shown in FIG. 6(a) shows an example of a pattern formed bylayering patterns each obtained by rotating one pattern at 60° and at120° on the original pattern having linear opening parts and madesymmetric three times. As a result, as shown in FIG. 6(b) which ismagnified, the opening parts of each hexagonal shape dispersedly existand are arranged at every 60° but do not form opening parts continuousonly in one direction.

As described above, since a wafer tends to be warped to result indeterioration of properties and damage at the time of handling in caseof growing a nitride type compound semiconductor layer in the thicknessof about 10 μm or more by the ELO growth on a sapphire substrate,inventors of the present invention have enthusiastically madeinvestigations in order to avoid warp of a wafer even in case of growingthick semiconductor layer, and found that there is correlation betweenthe warp and the direction of the opening parts to be a seed in themask.

That is, even in case of a semiconductor layer grown by the ELO growth,the threading dislocation of an underlayer is inherited on the openingparts, so that the threading dislocation remains in the semiconductorlayer at the portion corresponding to the opening parts, and it is knownwell that, in the center part of the mask where the layer grown in bothdirections is joined to itself, the dislocation density is increased. Soif it is no need to emit light in the whole area of a chip just in caseof a semiconductor laser but is sufficient to emit light only instripe-like part, it is preferable to include no such opening part andjoined part where the dislocation density is high in the light emittingpart in stripe. From a viewpoint of that, as shown in FIG. 9, thewarping degrees between the top part and the center (T-C) and betweenthe bottom part and the center (B-C) in FIG. 9 are 300 μm, respectively,in case of growing a 24 μm-thick GaN layer on a 2 inch wafer of a 330μm-thick sapphire substrate by ELO growth using a mask layer havingopening parts 44 only in only one direction on the wafer, whereas thewarping degrees between the left side and the center (L-C) and betweenthe right side and the center (R-C) are 40 μm, respectively, to make itclear that there is a close relation between the direction of theopening parts and the warping.

Incidentally, in case of ELO growth in 4 μm thickness, the warping isabout 40 μm in any direction, and in case of 700 μm thickness of thesubstrate, even if the growth thickness is 24 μm, the warping degrees inthe foregoing respective directions are 50 μm and 20 μm, respectively,however, the warping similar to that described above is caused by makingthe substrate thin to about 300 μm before the resultant substrate isfinally cut (cleaved) into chips. On the other hand, in case of ELOgrowth of a 24 μm-thick GaN semiconductor layer using the mask with thepattern shown in FIG. 6(a), the warping degrees are 60 μm in theforegoing (T-C, B-C) direction and (L-C, R-C) directions. That is, ifusing the mask layer having the opening parts in the pattern shown inFIG. 6(a), the warping degrees are slightly increased, however they aresignificantly decreased as compared with those in case of using aconventional mask layer having opening parts formed only in onedirection.

The pattern shown in FIG. 6(a) is formed by respectively overlaying afirst pattern having periodically formed linear pattern parts P with 20to 30 μm width and linear opening parts Q with 10 to 20 μm width (inFIG. 6, although P is shown narrower than Q, that is only to show theopening parts clearly and actually, P is 1 to 3 times as wide as Q), asecond pattern which is the first pattern rotated at 60 degrees, and athird pattern which is the first pattern rotated at 120 degrees and theresultant opening parts 3 a are hexagonal (not necessarily needed to beright hexagonal) as shown in FIG. 6(b). Since a nitride type compoundsemiconductor layer grown from the opening parts 3 a on the mask 3 inthe lateral direction is hexagonal system, the growth can be promoted ina constant ratio in the opening parts in any direction.

As a result, the joined part of the semiconductor layer grown in thelateral direction from a plurality of opening parts becomes point andthe dislocation density in the joined part is scarcely increased. Hence,as shown as S in FIG. 6(a), stripe type light emitting part with a lowdislocation density can be obtained by forming the stripe type lightemitting part on the part of mask 3 which is without transverselycrossing the opening parts 3 a and in the half region avoiding thecenter part of the mask width P.

The semiconductor laser chip (LD) having light emitting part in stripeformed in such portion where the dislocation density is extremely lowhas a similar structure as the foregoing structure shown in FIG. 1. Thatis, the semiconductor laser in this example, comprises a mask layer 3having opening parts 3 a formed directly on a substrate 1 or on a firstnitride type compound semiconductor layer 2 deposited on the substrate1, a second nitride type compound semiconductor layer 4 formed on themask layer 3 by being selectively grown in the lateral direction fromthe foregoing opening parts 3 a, and a semiconductor layered part 15composed of laminated nitride type compound layers as to form lightemitting layer having stripe type light emitting part. The mask layer 3is so formed as to be extended in the whole length of chips withouttransversely crossing the opening parts under the stripe type lightemitting part and the opening parts 3 a dispersedly exist in theportions other than the portion above which the stripe type lightemitting part is formed.

Regarding this example, the structures, the materials, and theproduction methods of the substrate 1, the second nitride type compoundsemiconductor layer 4, the semiconductor layered part 15, the contactlayer 13 formed thereon, the electrodes, and the like are same as thoseof the foregoing example shown in FIG. 1 except that the mask layer 3 isformed as to have the shape shown in FIG. 6(a) and therefore theirdescription is omitted.

Further, the laser structure may be formed to be a structure in whichonly the electrode is in stripe without etching the contact layer, be amesa type structure formed almost near to the active layer, be aproton-implanted type, or be another structure such as an index guidingstructure in which current blocking layer is formed.

According to the present example, since the nitride type compoundsemiconductor layer part with a small dislocation density is formedlinearly without transversely crossing the opening parts, and thesemiconductor layered part as to form a light emitting part in stripe isformed in the portion where the dislocation density is low, whileeliminating warping of the wafer by not forming the opening partscontinuous only in one direction, the stripe type resonance part wheresemiconductor laser is oscillated can be formed on the semiconductorlayer with excellent crystallinity and flatness and the semiconductorlayered part of the resonance part is also grown with excellentcrystallinity to give a semiconductor laser with a low threshold currentvalue and excellent properties. Moreover, the entire surface of thewafer can be utilized to avoid vain use of a half or more of thesubstrate. Further, by forming the recessed part on the surface of themask layer, no contact stress affect between the mask layer and thesemiconductor layer at the time of semiconductor layer growth and thecrystallographic axis of the semiconductor layer is not curved by thestress to grow a flat semiconductor layer in a wide width.

Other examples shown in FIG. 7 and FIG. 8 are mask patterns capable ofsuppressing the wafer warp while forming light emitting region with asmall dislocation density in the whole region in the stripe type lightemitting part of an LD without transversely crossing the openingportions with a high dislocation density. That is, in the example shownin FIG. 7 the linear mask layer 3 sandwiched between neighboring linearopening parts 3 d is the portion corresponding to the stripe type lightemitting part S and other parts of the mask layer 3 are formed in alattice-like shape and rectangular opening parts 3 a are formed like amatrix. As a result, in a wafer, although the linear opening partspenetrate the wafer in the stripe type light emitting part S, but theopening parts 3 a are formed in both vertical and lateral directions inother parts, so that warping does not take place only in one directionand warping of the wafer can be suppressed. Incidentally, although theintervals (the mask width) of the rectangular opening parts 3 a areshown at the same intervals in the vertical and lateral direction inFIG. 7, the intervals can be adjusted corresponding to the difference ofthe growth rate. Further, another linear mask part, which is formedadjacently to the stripe type light emitting part S, is to increase therate in the lateral direction in the periphery of the light emittingpart and it may be only one stripe type light emitting part S.

Whereas the foregoing example is an example in which the pattern of onechip is repeated in the same shape in a wafer (only the pattern of onechip is shown in FIG. 7), the example shown in FIG. 8 is an example ofpattern shutting the continuity among pattern of respective chips. Thatis, one chip has a structure in which opening parts are formed inparallel to the stripe type light emitting part S, however this patternis formed as to reverse the mask parts 3 and the opening parts 3 e inthe region J to be used as chips and the neighboring region K not to beused as chips. At the time of producing chips from the wafer, the waferis cut along the dotted line H to produce LD chips in the region J,whereas the region K is discarded. Also in this pattern, since theopening parts are not linearly continuous, the wafer warp can besuppressed. Incidentally, the region K is to be discarded and thereforepreferable to be short and even if the region K is shortened to be abouta half of the length of the region J, the warp of the wafer is scarcelyaffected. In case of an LED, since it is no need to cut, even the regionK can be used and the wafer is not at all used in vain.

According to the present invention in which recessed parts are formed inthe mask layer, even if the width of the mask is widened, thecrystallographic axis of the semiconductor layer selectively grown inthe lateral direction thereon is not curved and while keeping a lowdislocation density attributed to the selective growth in the lateraldirection, the flatness of the semiconductor layer can be maintained, sothat a nitride type compound semiconductor layer with excellentcrystallinity and flatness can be formed in a wide range and a nitridetype compound semiconductor light emitting device such as ablue-emitting type semiconductor light emitting device can be obtained.Especially, application of the present invention to a blue-emitting typesemiconductor laser using nitride type compound semiconductor makes itpossible to obtain a semiconductor laser with a low threshold currentvalue.

Further, according to the present invention in which the pattern of themask layer is innovative, since the opening parts of the mask layer arenot composed of opening parts continuous only in one direction, even ifthe mask width is widened and the thickness of the semiconductor layerto be grown thereon is required to be thick, no warp of the wafer iscaused. Consequently, a semiconductor light emitting device withexcellent quality can be obtained at a low cost without any bad effecton the handling and device properties in the wafer processing process.

Furthermore, in the semiconductor laser of the present invention,whilethe stripe type light emitting part does not at all include parts with alarge dislocation density and comprise the semiconductor layered partwith few crystal defects in the light emitting portion, there is nolinear opening part extended only in one direction in portions otherthan the peripheral parts of the stripe type light emitting part,therefore, even if a thick nitride type compound semiconductor layer isgrown, there takes place no problem in the wafer state. Moreover, thesubstrate can efficiently be used and the fabrication cost can belowered. As a result, a semiconductor laser with a low threshold valueand excellent properties can be obtained at a low cost.

Although preferred examples have been described in some detail it is tobe understood that certain changes can be made by those skilled in theart without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A method for manufacturing a semiconductor lightemitting device comprising the steps of: forming a mask layer, on whicha nitride type compound semiconductor layer is not directly formed,directly on the surface of a wafer type substrate or on a layer formedon said substrate, forming opening parts for exposing seeds to grow anitride type compound semiconductor layer on said mask layer in a mannerthat said opening parts are not arranged only continuous in one singledirection, forming a nitride type compound semiconductor layer on theentire face of said wafer type substrate by selective growth in thelateral direction on said mask layer from said opening parts, forming asemiconductor layered part composed of nitride type compoundsemiconductor layers as to form a light emitting layer on said nitridetype compound semiconductor layer, and making said wafer type substrateinto chips.
 2. The method of claim 1, wherein most of said opening partsare formed to be rectangular or hexagonal shape in a plan view.